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Helen Branswell
THE CANADIAN PRESS
SPEAK UP (Read in full DR Memoli states we need to think about using Peramivir sooner! Seriously sick patients Need Peramivir First!)
TORONTO - American scientists have reported the first known case of H1N1 viruses that are resistant to the newest flu drug to hit the market, peramivir.
The case involved a person with H1N1 flu who first developed resistance to oseltamivir or Tamiflu. Because of similarities in the drugs' chemical structures, it was expected that viruses resistant to Tamiflu would be resistant to the new drug as well.
But the confirmation makes it clear doctors treating severely ill flu patients need to try to maximize the benefits of the few flu drugs available and minimize the risk of resistance developing, said Dr. Matthew Memoli, a physician and researcher at the National Institute of Allergy and Infectious Diseases and first author of the report.
Memoli suggested doctors can do that by giving more thought to the order in which they use flu drugs when treating seriously sick patients or those who seem at risk of becoming gravely ill because they suffer from other medical conditions.
"If you have someone who's very sick "¦ and you're very worried about them, maybe they're on their way to the ICU or you think they could be heading that way "¦ maybe we need to go straight to peramivir," he said from Bethesda, Md.
The report, published in the journal Clinical Infectious Diseases, also suggests resistance to these drugs in immunocompromised patients may arise more rapidly than might have been previously thought.
Two immunocompromised patients, both of whom had undergone stem cell transplants, developed resistance to Tamiflu quickly after being put on the drug. In one patient, resistance developed somewhere between Day 5 and Day 9 of treatment while in the other it occurred sometime before Day 14.
About 22 per cent of the 267 reported cases of oseltamivir-resistant H1N1 flu worldwide have been seen in people who are immunocompromised.
These patients have a harder time fighting off their infection, so they are kept on flu drugs for substantially longer than is normally the case - sometimes weeks. The patient who developed resistance to peramivir, for instance, was given 24 days of Tamiflu treatment before being switched to peramivir.
The prolonged treatment creates a situation where the viruses that are vulnerable to the drugs die off, but those that can survive the attack replicate and flourish. This paper suggests that process can happen pretty quickly.
"While the emergence of drug-resistant influenza virus is not in itself surprising, these cases demonstrate that resistant strains can emerge after only a brief period of drug therapy," NIAID Director Dr. Anthony Fauci said in a release.
Using peramivir right from the start might be more effective, Memoli said. The drug is administered intravenously, a delivery mode that is more effective with severely ill patients because it gets more of the drug into the system. Tamiflu is administered in pill form; pills are not always fully absorbed in severe cases.
Hitting the infection with an intravenous drug from the start might vanquish the virus before resistance can develop, Memoli said, adding more study will be needed to see if this is a more effective regimen.
"Once the patient has developed resistance to oseltamivir, peramivir is unlikely to work," he said. "It's possible what we need is to think about using peramivir "¦ sooner."
So far all H1N1 viruses resistant to Tamiflu and peramivir have been susceptible to zanamivir (sold as Relenza). But that drug is inhaled. Inhalation is not a workable delivery mode for severely ill flu patients, especially those who are on a machine to help them breathe.
Fauci said the cases also underscore the need for more pharmaceutical weapons with which to fight flu.
"We have a limited number of drugs available for treating influenza and these findings provide additional urgency to efforts to develop antivirals that attack influenza virus in novel ways."
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